This is a phase III randomized, blinded study of nevirapine vs. nevirapine placebo for the prevention of maternal to fetal transmission in pregnant women who present during the third trimester of pregnancy. Participating mothers will be given a single blinded dose during active labor. Neonates will be given a single dose of nevirapine elixir or placebo elixir by mouth at 2-3 days of life. The primary endpoint is incidence of transmission of HIV infection from mother to fetus. The secondary endpoints are monitoring of maternal and neonatal toxicities, determination of nevirapoine pharmacokinetics in neonates, determinine the relationship between maternal plasma viremia and risk of vertical transmission and the effect on neonate plasma viremia.